A lithium ion battery is a secondary battery having a structure in which lithium dissolves out as ions from a positive electrode at the time of charging and moves to a negative electrode to be stored therein, and conversely, the lithium ions return to the positive electrode from the negative electrode at the time of discharging. Since the lithium ion battery has features such as high energy density and a long life cycle, it is widely used as a power supply for electric appliances such as a video camera, portable electronic devices such as a laptop computer and a mobile telephone, electric tools such as a power tool, and the like. Recently, the lithium ion battery is also applied to a large-sized battery that is mounted in an electric vehicle (EV), a hybrid electric vehicle (HEV), and the like.
This kind of a lithium ion battery is constituted of a positive electrode, a negative electrode, and an ion conducting layer inserted between both of the electrodes. As the ion conducting layer, a separator constituted of a porous film, such as polyethylene and polypropylene, which is filled with a non-aqueous electrolytic solution, is generally used. However, since such an organic electrolytic solution using a flammable organic solvent as a solvent is used as an electrolyte, it is required to improve a structure and material for preventing volatilization or leakage, and also, it is required to install a safety device for suppressing an increase in temperature at the time of a short circuit and to improve the structure and material for preventing a short circuit.
In contrast, an all-solid lithium secondary battery that is constituted by solidifying the whole battery using a solid electrolyte does not use a flammable organic solvent, and thus, it is possible to attempt the simplification of a safety device, and also, the battery can be made excellent in the production cost or productivity. In addition, it is possible to attempt high voltage by laminating the solid electrolyte in a cell in series. Furthermore, for this kind of a solid electrolyte, since elements do not move except Li ions, there are no side reactions by the movement of anion, and thus, it is expected that it leads to improve safety or durability.
Such a solid electrolyte that is used for a battery is required to have high ionic conductivity, and to be chemically or electrochemically stable. For example, lithium halide, lithium nitride, lithium oxyacid salt, or derivatives thereof are known as a candidate of the raw material.
With regard to this kind of a solid electrolyte, for example, Patent Document 1 discloses a sulfide-based solid electrolyte, in which a high temperature lithium ion conductive compound composed of lithium phosphate (Li3PO4) allows to be in lithium ion conductive sulfide glass represented by a general formula, Li2S—X (wherein X represents at least one sulfide of SiS2, GeS2, and B2S3).
In addition, Patent Document 2 discloses a sulfide-based solid electrolyte comprising a lithium ion conductive material that is a composite compound represented by a general formula, Li2S—GeS2—X (wherein X represents at least one of Ga2S3 and ZnS) as a material that is crystalline and exhibits very high ionic conductivity, that is, 6.49×10−5 Scm−1 of the ionic conductivity at room temperature.
Patent Document 3 discloses a lithium ion conductive sulfide ceramic, which is a sulfide ceramic having high lithium ionic conductivity and decomposition voltage, comprising Li2S and P2S5 as a main component, and having compositions of Li2S=82.5 to 92.5 and P2S5=7.5 to 17.5 in mol %, and among them, preferably a composition of Li2S/P2S5=7 (composition formula: Li7PS6) in molar ratio.
Patent Document 4 discloses a lithium ion conductive material that is represented by a chemical formula: Li+(12-n-x)Bn+X2−(6-x)Y−x (wherein Bn+ represents at least one selected from P, As, Ge, Ga, Sb, Si, Sn, Al, In, Ti, V, Nb, and Ta, X2− represents at least one selected from S, Se, and Te, Y− represents at least one selected from F, Cl, Br, I, CN, OCN, SCN, and N3, and 0≦x≦2), and has an argyrodite-type crystal structure.
Patent Document 5 discloses a lithium argyrodite, which is a solid compound capable of being prepared as a single layer in addition to high fluidity of the lithium ion, and is represented by a general formula (I) Li+(12-n-x)Bn+X2−(6-x)Y−x, wherein, in the formula, Bn+ is selected from the group consisting of P, As, Ge, Ga, Sb, Si, Sn, Al, In, Ti, V, Nb, and Ta, X2− is selected from the group consisting of S, Se, and Te, Y− is selected from the group consisting of Cl, Br, I, F, CN, OCN, SCN, and N3, and 0≦x≦2.
Patent Document 6 discloses a sulfide-based solid electrolyte, which is a new sulfide-based solid electrolyte capable of significantly enhancing conductivity compared with conventional solid electrolytes, having a framework structure of Li7PS6, and comprising a composition formula: Li7+xP1-ySiyS6 (wherein x is in a range of −0.6 to 0.6 and y is in a range of 0.1 to 0.6) in which a part of P is substituted by Si.